Helical buckling thwarts the efforts of many who aspire to resolve wellbore or pipe problems with mechanical equipment that utilizes a long, flexible rod or tube. Coiled tubing operations (CT) especially encounter helical buckling problems when the tubing is of extended length in deviated wellbores. This problem often limits the extent of reach in extended reach coiled tubing operations. Coiled tubing may experience helical buckling as the tubing travels through high friction regions of a wellbore or through horizontal regions of a wellbore. In conventional coiled tubing operations, the tubing is translated along the borehole either via gravity or via an injector pushing from the surface. For an extended reach horizontal wellbore, an axial compressive load will build up along the length of the coiled tubing due to frictional interactions between the coiled tubing and the borehole wall. A typical axial load 100 as a function of measured depth 102 is plotted in FIG. 1. This wellbore has a 4000 foot vertical section, a 600 foot, 15 degree per 100 foot dogleg from vertical to horizontal, and then continues horizontal until the end.
If the horizontal section of the wellbore is sufficiently long, the axial compressive load 100 will be large enough to cause the coiled tubing to buckle. The first buckling mode is referred to as “sinusoidal buckling”—in this mode, the coiled tubing snakes along the bottom of the borehole with curvature in alternating senses. This is a fairly benign buckling mode, in the sense that neither the internal stresses nor frictional loads increase significantly. As the axial compressive load 100 continues to increase, the coiled tubing will buckle in a second buckling mode. This buckling mode is called “helical buckling”—this mode consists of the coiled tubing spiraling or wrapping along the borehole wall. This buckling mode can have quite severe consequences—once the coiled tubing begins to buckle helically, the normal force exerted by the borehole wall on the tubing increases very quickly. This causes a proportional increase in frictional loading, which in turn creates an increase in axial compressive load 100. Once helically buckling has initiated, the axial compressive load 100 increases very quickly to a level such that the tubing can no longer be pushed into the whole. This condition is termed “lock-up.” A plot of axial stress 200 as a function of measured depth 202 for a coiled tubing which is almost in a locked up state is shown in FIG. 2.
Coiled tubing (CT) operations employ several techniques for maximizing the depth of penetration in extended reach wells. Vibrators are used in conjunction with CT to increase the depth of penetration in extended reach wells. These vibrators are made up to the bottomhole assembly (BHA) connected at the end of the CT string and are normally activated by pumping fluid through them. The oscillating action caused by the vibrator results in reduced drag forces on the pipe as it is pushed into the wellbore from the surface. One of the more effective solutions uses a vibrator as part of the bottomhole assembly (BHA). The oscillations caused by the vibrator reduce the excessive drag on the CT string in high angle wellbore trajectories. This reduction in drag often delays the onset of helical buckling. Effectively, this drag reduction has been found to be equivalent to as much as 30% of the friction coefficient between the wellbore wall and the CT. Thus, drag force reduction increases the CT's ability to go further in an extended reach well. However, depending on the wellbore configuration and the CT string characteristics, as well as the vibrator's amplitude and frequency of the oscillations produced, the position of the vibrator at the terminal end of the BHA may not be effective to allow well total depth (or target depth) to be reached.
When a CT string goes into lockup mode, the entire string length is not completely helically-buckled. There are typically one or two locations in the wellbore where the CT is at a critical state, depending on several physical factors, including wellbore/completion design, CT string characteristics, etc. Lock-up developing in these one or two critical locations is sufficient to prevent the CT from advancing further into the wellbore. The location is typically either near surface below the wellhead for most high angle wells or near the heel of a long horizontal well or both. These locations can be identified prior to actual insertion of the CT into the well through analysis using a force modeling software such as COILCADE™, a commercially available product available from Schlumberger Technology Corporation.
Similarly, pipe used to connect the output of wellbores in oil fields including offshore operations may require maintenance to remove residue and/or improve flow. Such systems exercise flexible tubing equipment that experiences similar buckling along the length of the tubing when equipment is introduced to service the pipelines.